Transformation of Shuttle Radar Topography Mission (SRTM) Digital Elevation Data to Nigerian Height System
Determination of height information using the classical field surveying and geodetic methods is rather expensive, rigorous and time consuming. It is also limited in the capacity of the earth surface data gathered. These conventional topographic mapping technologies have produced maps with a variety of scales and of uneven quality - some with astounding accuracy, some far less adequate. A good alternative is the Shuttle Radar Topography Mission (SRTM) derived Digital Elevation Model (DEM) which provides an excellent base data for extracting heights for topographic mapping. This will ameliorate the present situation. The SRTM space mission produced the most complete, highest resolution digital elevation model of the Earth. This study is aimed at developing a model for the transformation of the SRTM heights from WGS84 datum to the Nigerian height system (Minna datum) using SRTM derived DEM, topographic map and Global Positioning System (GPS) data of some parts of Lagos state, Nigeria. The chosen site corresponds to the area covered by one map sheet at the scale of 1:25,000 (13.8km x 13.8km) which is 190.44sq.km. Software such as Globalmapper, Surfer 8.0 and ArcGIS 9.3 were used for specialized data processing and analysis. However, the datasets obtainedwere first projected to a common system and subsequently harmonized. This was carried out with an overlay of GPS points on the grid based SRTM and topographic DEM surfaces of the environment. Results showed deviation in heights with coinciding planimetry data. The average absolute error of the SRTM DEM for our test site was gotten as +/-0.22087m. Finally, by modelling this vertical shift, a transformation model that is accurate with a standard error of 0.238m was developed.